Water may become contaminated with organic compounds in many ways. For example, ground water may be polluted by discharges from industrial processes or accidental spills. Water condensed from closed atmospheres, such as those found in spacecraft, is frequently contaminated with small amounts of organic compounds which have collected in the atmosphere. Regardless of the source of organic contamination, it is often desirable to completely remove the organics to make the water potable, especially in a closed environment like a spacecraft in which it is desirable to recycle all water.
C.sub.4 and larger organic compounds are relatively easy to remove from water by conventional methods such as adsorption or ion exchange. These removal methods, however, tend to leave low concentrations of organic compounds, especially low molecular weight compounds, in the water. In this context, low molecular weight compounds means C.sub.3 and smaller organic compounds, including compounds which contain nitrogen, sulfur, halogen, or other atoms in addition to carbon, hydrogen, and oxygen atoms. Low concentration means concentrations less than about 500 parts per million (ppm). Low concentrations of low molecular weight organic compounds are difficult to remove from water streams, particularly those compounds which do not ionize. In addition, such compounds are not readily adsorbed on physical sorbents like activated charcoals, zeolites, and other high surface area sorbents. Oxidative processes which can remove aqueous organic contaminants are known, but they have drawbacks which make them unsuitable for use in closed environments or situations in which portability of the decontamination system is desirable. The key requirements of a process suitable for use in closed environments or situations requiring portability include high reliability, simplicity, low weight, low system volume, low power consumption, and low expendables consumption. These requirements can also be expressed as a need for a short residence time, which will minimize system weight and volume, and low reaction temperature and pressure, which will minimize system weight and power consumption.
The classic process for oxidatively treating contaminated water, known as wet air oxidation, is described in U.S. Pat. No. 2,665,249 to Zimmermann and U.S. Pat. No. 3,133,016 to Stine et al. The Zimmermann process requires temperatures of at least 450.degree. F. and the Stine process requires temperatures between 200.degree. F. and 600.degree. F. and elevated pressures. Although both processes are capable of reducing the total organic carbon (TOC) content of contaminated water, they are incapable of removing some low molecular weight organic compounds, especially acetic acid, from the water. Moreover, the processes are energy intensive and require reactors constructed from materials like titanium to withstand corrosive operating conditions.
Supercritical water oxidation is capable of converting aqueous organics, including acetic acid, to their elemental oxides. The process requires temperatures between 750.degree. F. and 1200.degree. F. and pressures between 3200 psig and 4000 psig. These high temperatures and pressures require vessels made with thick walls and costly alloys. As a result, the components of a supercritical water oxidation system are heavy and power consumption is high.
Photochemical processes which make use of light, such as ultraviolet or sunlight; a chemical oxidant, such as hydrogen peroxide or ozone; and sometimes a catalyst, such as titanium oxide, can also convert aqueous organics to their elemental oxides. However, these processes tend to have low conversions and require recycling to obtain complete oxidation of the organic contaminants. Reactor residence times of 30 minutes or more can be required. The large reactors required to achieve such residence times and the power demands imposed by the recycle make such processes impractical for use in closed environments or portable decontamination systems.
A catalytic oxidation process which uses a combination of absorptive beds and noble metal catalysts on activated alumina or similar supports to reduce the chemical oxidation demand (COD) of waste water is able, in some instances, to reduce the COD to zero. However, the process cannot routinely achieve a complete removal of COD. In the instances in which complete removal is possible, such results are only achievable with fresh catalyst. After only a few hours of operation, the process achieves a steady state effluent COD of several hundred milligrams per liter (mg/l).
Other catalytic oxidation studies with low molecular weight aqueous organic compounds have shown incomplete conversion at residence times of 3 to 5 minutes or more, even at reactor temperatures greater than 500.degree. F. and pressures above 1000 psig.
Accordingly, it would be desirable to have a means for completely removing low concentrations of low molecular weight organic contaminants from water streams with a short residence time, a low reaction temperature, and a low reaction pressure.